Most paper or paperboard operations discharge tremendous quantities of liquid which contain suspended solids, such as wood fibers, clay, calcium carbonate, titanium dioxide, sand, and wood bark, along with oxygen demanding organic compounds, such as starches, lignins, and hemicellulose. In addition, the liquid discharge can also contain trace chemicals, such as chlorinated organic compounds, polychlorinated biphenols, metals such as barium, aluminum, and other trace elements. The adverse environmental impact of this liquid discharge from a papermaking operation is well documented.
There are several reasons for the liquid discharge from a papermaking operation. In some cases, the liquid discharge is due to careless, excessive use of water and, if excess water is used, it must be discharged from the operation. In other cases, the liquid discharge is the result of poor water system design, which results in inadvertent use of fresh water. In still other instances, excess water has been employed in an attempt to keep the suspended and dissolved solid levels at a low enough level to ensure optimum paper machine conditions. In this latter case, the added water effects a reduction in suspended and dissolved solids, but produces a corresponding increase in effluent flow.
Typically the most highly contaminated water discharged from a conventional papermaking operation utilizing recycled waste paper, results from the cleaning of the recycled pulp. The liquid residue from the cleaning operation is normally subjected to floatation clarification, in which the lighter weight solid material is recovered as a floating layer which can be burned or landfilled. The liquid effluent from the clarification process, which contains dissolved solids and oxygen demanding compounds, then must be treated by either an aerobic or anaerobic process before it can be discharged to the sewer system. Evaporation of the liquid effluent has not been feasible due to the tremendous volume of effluent.